Electrical generators are used in a wide variety of applications. Typically, an individual electrical generator operates in a stand-by mode wherein the electrical power provided by a utility is monitored such that if the commercial electrical power from the utility fails, the engine of the electrical generator is automatically started causing the alternator to generate electrical power. When the electrical power generated by the alternator reaches a predetermined voltage and frequency desired by the customer, a transfer switch transfers the load imposed by the customer from the commercial power lines to the electrical generator. As is known, most residential electric equipment in the United States is designed to be used in connection with electrical power having a fixed frequency, namely, sixty (60) hertz (Hz).
Typically, electrical generators utilize a single driving engine coupled to a generator or alternator through a common shaft. Upon actuation of the engine, the crankshaft rotates the common shaft so as to drive the alternator that, in turn, generates electrical power. The frequency of the output power of most prior electrical generators depends on a fixed, operating speed of the engine. Typically, the predetermined operating speed of an engine for a two-pole, stand-by electrical generator is approximately 3600 revolutions per minute to produce the rated frequency and power for which the unit is designed.
The output power of a generator is, however, load dependent. As the electrical load on the generator increases, the frequency of the electricity output may decrease. Further, a sudden, significant change in load can cause the generator speed to drop several hertz, for example. If the generator has sufficient capacity to power the additional load, the generator controller regulates the speed and brings the output frequency back to the desired output frequency. If, however, the additional load exceeds the capacity of the generator, the generator may stall, resulting in none of the loads receiving power. For example, a generator may be running and providing power to lights and a television in one room. A sump pump, furnace, or another electrical load may be switched on, creating an additional power demand on the generator. The additional load may cause the engine to begin to slow and/or to stall.
Historically, it has been known to provide a load shed module which may be connected to the output of a transfer switch. The load shed module, therefore, receives power from the generator as an input. One or more sensors are utilized within the load shed module to monitor operation of the generator. Optionally, the load shed module may receive one or more signals from the generator as inputs providing an indication of the operation of the generator. The signals may indicate, for example, that the generator is providing power to the loads rather than utility power or that the generator is overloaded. The load shed module also includes a number of switches or contactors to connect load circuits to the generator. A central controller receives the signals from the sensors monitoring operation of the generator or the signals from the generator to determine whether the load circuits may be connected to the generator. If the generator begins to stall, the load shed module may disconnect a portion, or all, of the loads according to the load management routines programmed into the load shed module. However, such a load shed module requires a central controller interconnected with each of the switches. Further, installation of the load shed module requires routing the electrical conductors from all of the loads to the load shed module. Each load typically includes a hot conductor and a neutral conductor and may further include a ground conductor. Each of the electrical conductors must then be properly connected within the load shed module. Depending on the arrangement of the load shed module, only the hot conductors may be switched or both the hot and neutral conductors may be switched. Further, each load must be connected to the appropriate switch designated for that load. The varying potential configurations and multiple conductors from each of the loads increase the potential for a wiring error to occur during installation of the load shed module.
Therefore, it is a feature of the present invention to provide a load shed module with reduced installation complexity.
According to one embodiment of the invention, a load shed module is configured to be connected in series between a power source and a load. The load shed module includes an input configured to receive a first electrical connection from the power source, an output configured to provide a second electrical connection for the load, and a switch operatively connected in series between the input and the output. The switch receives a control signal to selectively open and close the switch. A sensor is configured to generate a feedback signal corresponding to an alternating current (AC) voltage present at the input, and a priority selector is operable to generate a priority signal for the load shed module. A controller is connected to the sensor to receive the feedback signal, to the priority selector to receive the priority signal, and to the switch to supply the control signal. The controller is operable to determine whether the power source is a utility power source or an alternate power source as a function of the feedback signal, compare the feedback signal to a threshold when the power source is the alternate power source, generate the control signal to open the switch when the feedback signal exceeds the threshold for a first predetermined time, and generate the control signal to close the switch when the power source is the alternate power source and when a second predetermined time has passed, where the second predetermined time is selected as a function of the priority signal.
According to another aspect of the invention, the controller is further operable to determine a frequency of the AC voltage present at the input from the feedback signal and determine whether the power source is the utility power source or the alternate power source as a function of the frequency of the AC voltage. The controller may store a minimum and maximum frequency of the AC voltage and determine a difference between the minimum and maximum frequencies. The threshold to open the switch may be a maximum difference between the minimum frequency and the maximum frequency. Optionally, the controller may determine a difference between the frequency of the AC voltage present at the input and an expected frequency of the AC voltage present at the input. The threshold may be a maximum difference between the frequency of the AC voltage present at the input and the expected frequency of the AC voltage present at the input.
According to yet another aspect of the invention, the controller may be configured to operate in a first operating mode and in a second operating mode. During the first operating mode, the controller generates the control signal to open the switch when the feedback signal exceeds the threshold for the first predetermined time. During the second operating mode, the controller generates the control signal to open the switch when the feedback signal exceeds the threshold for a third predetermined time, and the third predetermined time is less than the first predetermined time. The controller may also enter a lock out operating mode when the feedback signal exceeds the threshold for the third predetermined time.
According to another embodiment of the invention, a method of connecting a load to a power source in a power distribution system is disclosed. A feedback signal, corresponding to a voltage received at an input of a load shed module from the power source, is read. A frequency of the voltage is determined from the feedback signal with a controller in the load shed module and compared to a threshold frequency. A control signal is generated with the controller to open a switch connected in series between the input and an output of the load shed module when the frequency of the voltage at the input is outside of the threshold frequency for a first predetermined time. A priority level of the load shed module is determined and the control signal is generated with the controller to close the switch connected in series between the input and the output of the load shed module after a second predetermined time when the frequency of the voltage is within, the threshold frequency, where the second predetermined time is a function of the priority level of the load shed module.
These and other objects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.